The spinal column of bones, or spine, is highly complex in that it includes over twenty vertebral bones, called vertebrae, coupled to one another so as to support the body and to house and protect critical elements of the nervous system. In addition, the spine is a highly flexible structure, capable of a high degree of curvature and twist in multiple directions. The most flexible of all the regions of the spine is the cervical spine. The bones and connective tissue of an adult human spine are coupled sequentially to one another by a tri-joint complex which consists of an anterior disc and the two posterior facet joints. The vertebral bodies of the vertebrae are separated and cushioned by soft yet resilient spacers referred to as intervertebral discs. The vertebral bones of the spine are classified as cervical, thoracic, lumbar, and sacral, and defining vertebral levels, e.g., L3 for the third lumbar vertebral level. The cervical portion of the spine, which comprises the upper portion of the spine up to the base of the skull, includes the first seven vertebrae. The twelve intermediate bones comprise the thoracic vertebrae, and connect to the lower spine which comprises the five lumbar vertebrae. The base of the spine is the sacral bones (including the coccyx).
Genetic or developmental irregularities, trauma, chronic stress, tumors, and disease, such as osteoporosis or degenerative disc disease (DDD), are a few of the causes which can result in spinal pathologies for which treatment procedures and assemblies have been disclosed in the art. These procedures and assemblies may be classified as anterior, posterior, or lateral. As the classification suggests, posterior procedures and assemblies are either attached to, or access to the vertebral bodies is achieved from, the back of the spinal column (transpedicular) or the side of the spinal column (extrapedicular). Such procedures and assemblies may include, by way of example, immobilization (fusion) of multiple vertebrae with instruments, such as rods and screws, introduction of balloon-expandable stents into one or more vertebral bodies to restore the loss of height of fractured vertebral bodies, or the introduction of bone cement into one or more vertebral bodies to fix and stabilize traumatic, osteoporotic and pathological fractures of the vertebral bodies and thus relieve pain. In each case, the hard cortical bone is penetrated to gain access to the vertebral body cavity via a surgical instrument.
The typical vertebra includes two parts: an anterior (or front) segment, which is the vertebral body; and a posterior (or back) segment called the vertebral (neural) arch, which encloses the vertebral opening where the spinal cord is housed. The vertebral arch is formed by a pair of pedicles and a pair of laminae, and supports seven processes, four articular, two transverse, and one spinous. The pedicles are two relatively short, thick processes, which project dorsally (e.g., sideways) one on either side, from the upper part of the vertebral body at the junction of the posterior and lateral surfaces of the vertebral body. The pedicles function to connect the vertebral body to the vertebral arch. The pedicles are often used as a radiographic marker and entry point in a variety of spinal surgical procedures, and include a relatively thick layer of dense and hard cortical bone to provide structural strength to the vertebral arch.
A trend in spinal surgery is to perform surgery in a minimally invasive or minimal access fashion to avoid the considerable trauma of so-called open or “direct access” procedures that require large incisions to lay bare the lesion space to be treated. While significant strides are being made in this area of minimal invasive surgery (MIS) and percutaneous procedures, a risk exists (as it does in open procedures) that the pedicle may become breached, cracked, or otherwise compromised during a procedure. One period during a posterior procedure in which the risk of a pedicle breach is significant is during the process of initially accessing the pedicle.
Options for achieving initial access to a pedicle include the use of a guide wire, such as a Kirschner wire (or K-wire), or a trocar, by way of example. Each implement has its advantages and disadvantages. With respect to use of a K-wire, which is typically made from heavily cold drawn high-strength implant steel (such as 316L), this procedure includes inserting the K-wire to the target site and driving the K-wire into the pedicle, creating a guide for larger caliber instruments to slide on, such as a cannula or cannulated trocar. The main advantage of any K-wire is its relatively small diameter, typically in the range of 0.8-2.4 mm, that allow safe and easy repositioning in case of initial misplacement, however slight, by retracting the K-wire back to the bony surface, re-adjusting its angle of introduction, and piercing the cortical bone another time, taking a new and different trajectory through the pedicle (typically under the mandatory guidance of C-Arm X-ray fluoroscopy). While surgical procedures in the lumbar or thoracic spine call for K-wire diameters in the 2 mm range, cervical spine procedures call for K-wire diameters in the 1 mm range, due to the smaller dimensions of most features of the cervical vertebrae. The pedicle has been left relatively undamaged due to the small diameter of the K-wire. Due to the size and shape of the typical K-wire, notably the absence of any handle affixed to its proximal end, manipulation and maneuvering of the K-wire may be cumbersome or difficult, increasing the risk of breaching the anterior cortical wall, nerves, or large blood vessels (e.g., the aorta and the vena cava). Especially when guiding larger caliber instruments, such as a cannula or a cannulated trocar over the K-wire, the relatively large surface area between the outer diameter of the K-wire and the inner diameters of the larger caliber tubular instruments often leads to frictional resistance in pushing the entire instrument assembly further distally, or anteriorly, towards the vertebral body, increasing the risk of inadvertently pushing the K-wire forward towards the anterior wall of the vertebral body. This risk is exasperated when the K-wire is not straight anymore, but slightly bent inside the cannulated instruments sliding on the K-wire. On the other hand, K-wires have also been known to protrude posteriorly out of the larger caliber tubular instruments and pierce the surgeon's surgical gloves, potentially injuring or infecting the surgeon.
A trocar has the advantage of being a one-step system and having its metal shaft affixed to an ergonomic plastic handle. However, because of its relatively larger diameter, more force is generally required to cause the trocar to pierce the cortical wall of the vertebral body, especially the pedicle that represents the longest distance of dense bone to go through (typically in a range of 2-50 mm, depending on the vertebral level) and is considered to be the strongest part of the vertebra. As such, a risk exists of breaking too much cortical bone (e.g., high risk of damaging or destroying the pedicle) when attempting to have the large caliber trocar eventually reach the cancellous bone inner region of the vertebral body and position any cannula or working sleeve for implant delivery. The risk of pedicle damage is exasperated when the user is trying to re-adjust the trajectory of the trocar either while the trocar is in the pedicle or by retracting the trocar to the bony surface, then re-adjusting the angle (typically under the guidance of C-arm X-ray fluoroscopy) before embarking on the new trajectory through the pedicle. Also, when the piercing tip of the trocar breaks through the cortical wall, resistance to penetration suddenly significantly decreases and the tip may breach the anterior cortical wall or damage the pedicle and/or adjacent nerves or blood vessels.
Another known surgical technique is the Jamshidi bone marrow biopsy technique, which uses special instruments called Jamshidi needles to pierce a cortical bone and to extract bone marrow from the bone. Jamshidi needles have found wide use in other surgical techniques, such as pedicle screw placement, for example.
Jamshidi needles may be used to locate the pedicle and the screw entry point and to break the cortex of the pedicle. A K-wire may be inserted through the Jamshidi hollow needle into the pedicle and the vertebral body, the Jamshidi needle acting as a guide tube for the K-wire. Cannulated screws can then be inserted over the K-wire safely, the K-wire acting as a guide wire for the cannulated screws. In the cervical spine lateral mass screws or hooks are more common, because pedicle screw placement in the cervical spine is challenging. The pedicles are rather narrow and therefore landmark identification and visualization is challenging when inserting pedicle screws. Different techniques can be used for pedicle screw placement in the cervical spine. Techniques are going from K-wire placement first under fluoroscopic control, to direct drilling with or without navigation. Due to the small screw diameter, the possible cannulation of the screw is rather small, and therefore the K-wire used typically has a small diameter as well. K-wires with small diameters bend easily and are difficult to guide. If the K-wires are bent, there is an increased risk of pushing the K-wire forward too much with the following surgical steps.
As an alternative technique pedicle probes and drills can be used. But because bigger diameters probes and drills are used and the pedicles are rather narrow in the cervical spine, this is very demanding without navigation.
In the lumbar and thoracic spine Jamshidi needles are often used to prepare the pedicle prior to screw insertion or vertebral augmentation procedures such as bone cement vertebroplasty, balloon kyphoplasty, or vertebral body stenting. Due to the narrow pedicles in the cervical spine the Jamshidi needles bear the risk of destroying the pedicle when pushing the Jamshidi needle into the bone. If an asymmetrical (e.g. beveled) tip is used, the Jamshidi needle also tends to go to the side and may over penetrate or penetrate out of the pedicle.